In U.S. Pat. No. 3,881,490 of May 6,1975, to Whitehead et al there is described a very thin absorbent pad for feminine hygiene, which is intended for use as an undergarment protector between menstrual periods as well as for absorbing light flow between and during periods.
The pad as described in the patent is comprised mainly of a thin air-formed batt of short-fibered wood pulp fluff. The batt is held together primarily by hydrogen bonds provided by spaced line embossments extending generally in the longitudinal direction and intersecting the ends and edges of the batt at spaced intervals. The ends and edges are otherwise unbonded and the resulting quilt-like pillows between embossments provide the finished pad with a cushioned top and soft, flexible borders.
As indicated earlier, while such pads are intended primarily for use for absorbing light flows, or for absorbing discharges to protect undergarments from being soiled between periods, many users found the pads to be so convenient and confortable that they also employed the pads for use in absorbing all menstrual fluids during regular periods. When thus employed, it was observed that while the pads did absorb surprisingly large amounts of fluids there was a tendency for them to become soaked rapidly, permitting fluids to reach the ends and sides and causing some garment soiling. To prevent the latter situation it was necessary to make many inconvenient pad changes at short intervals. The thin pads also tended to have a wet surface feel when sudden gushes occurred, even when fluids from such gushes were eventually drawn away by the embossed lines and redistributed throughout unembossed portions of the pad.
In spite of these above-noted inconveniences, many users found these thin pads to be sufficiently confortable that they often employed the pads even during times of relatively heavy menstrual flow. Because the thin pads had these desirable characteristics, many consumer's expressed an interest in obtaining pads of similar comfort but with higher absorbent capacity.
In response to this show of interest, many attempts were made to develop a pad which would fill that need while retaining the desirable features of the thin pad, i.e. a low cost pad of unitary structure, having the same soft edges and cushioned surface, but with much higher capacity for discharged body fluids.
When attempts were made to increase capacity by making the entire pad thicker while retaining the same embossed structure as the original thin pad, it was found that the embossing pressures needed to obtain sufficient fiber bonding to arrive at the desired unitary structure gave the resulting product a stiff, boardy character which was not comfortable, which did not conform well to body contours, and which did not seat itself well in undergarments. Transfer of fluid to the sides and ends of the pad still occurred after heavy flow, and the full potential capacity of the pad was not utilized to the desired extent. The wet surface feel after long wear or sudden gushes still remained. When less pressure was used in embossing to prevent the stiff, boardy properties, the resulting pad delaminated easily during handling and use, to make it unsuitable as a potential commercial product.
In another attempt to solve the above-mentioned problems, a contoured batt was constructed having more fiber provided in the central portion to obtain a thicker center. This batt was subsequently embossed in the same manner as the original thin pads. However the variance in thickness of the starting batt did not permit good embossing without applying excessive pressure, and when the necessary high pressures were used the disadvantage of a boardy product again emerged, particularly in the thicker central portion.
In still another attempt to obtain a satisfactory product, a separately formed thin batt was embossed as in the prior art and then laminated by adhesive means to an underlying and thicker unembossed batt which was also separately formed. With this product, problems were encountered first in getting a good bonded structure in the unembossed batt without using supplementary internal adhesive. The latter created stiffness and reduced the rate of fluid penetration. Second, the resulting edges were found to be too thick, and conformability was poor.
Finally it was found that by essentially using the same basic structure as the thin pad of U.S. Pat. No. 3,884,490 for the top layer of a composite structure, deforming a central portion of that thin pad upwardly, inserting a separate thicker pad inside the hump thus formed, and adhering the outer undeformed border portion of the uppermost layer and the bottom of the insert pad to a bottom supporting layer, that a product with high capacity, with soft edges, with a dry surface feel during use, and with good conformability was obtained.
The invention described herein is directed to a sanitary napkin of the latter improved structure.